we've changed the chainset and the bottom bracket on a 2006 trek 4300.
The chainset was a Suntour set and the ring wore out so we changed the set.
The bottom bracket was 124.5 and was also shagged.
The new cranks were unbranded copies and the new bottom bracket was 110.5 which kinda worked until that BB went south.

with the 11.5 the front mech never managed to get all three rings working(too close to the frame) so changed the new bottom bracket to a 113.5 unit as a compromise.

Now we can't reach to top ring!

How do i work out what size we need or diagnose whats wrong.

Thanks for reading. And I hope someone can help.

Mike

this may help;

Park Tools

Lots of cheap cranksets sit further out from the BB. You may need a shorter axle….

It does in terms of setting it up. Which we've done a fair few time. The origional cranks used a bottom bracket that was huge 124.5.
Its chainline that appears to be out.